1. Field of the Invention
The invention relates to stereoisomerically enriched diamines, to metal complexes comprising these diamines and also to their use in a process for asymmetrically reducing ketones using silanes, in particular polymethylhydrosiloxane, as reducing agents.
2. Brief Description of the Prior Art
As a consequence of the steadily growing significance of stereoisomerically enriched alcohols as fine chemicals, active ingredients in agrochemicals and pharmaceuticals or their intermediates and also their aromas and scents, the asymmetric reduction of ketones has found technical use on the industrial scale.
The focus is directed in particular to the development of catalysts which facilitate high yields and conversion rates at high stereoselectivities. U.S. Pat. No. 5,227,538 and J. Am. Chem. Soc. 116 (1994), 11667 disclose processes in which the asymmetric reduction of ketones using silanes in the presence of a titanium complex are carried out using optically active ligands as catalysts. A disadvantage of these processes is the high cost of the catalysts required in a large amount, which is in the range of 5 mol %, based on the ketone used.
WO-A 99/50211,WO-A 99/12877 and J. Am. Chem. Soc. 1999,121, 6158 disclose processes for asymmetrically reducing ketones which use polymethylhydrosiloxane (PHMS) as an inexpensive reducing agent and metal hydrides as catalysts, which are formed from a metal salt or a metal complex by a reducing agent. The asymmetric induction is effected by stereoisomerically enriched, secondary 1,2-diamines as ligands. A disadvantage of this method is the fact that this process only achieves moderate optical yields of the alcohols of 70 to 80%.
There was therefore a need to develop further catalysts and ligands which facilitate the asymmetric reduction of ketones in good yields and good optical purities.